1. Field of the Invention
Embodiments of the present invention relate to a liquid crystal display device (LCD), and more particularly, to an array substrate for the LCD including an organic thin film transistor (TFT) and a method of fabricating the same.
2. Discussion of the Related Art
Generally, an LCD device uses optical anisotropy and polarization properties of liquid crystal molecules to display an image. The liquid crystal molecules have an alignment direction along their lateral and bilateral axes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field to the liquid crystal molecules. In other words, as the intensity of the electric field is changed, the orientation of the alignment direction for the liquid crystal molecules also changes. Since incident light through liquid crystal molecules is refracted based on the orientation of the liquid crystal molecules, due to the optical anisotropy of the aligned liquid crystal molecules, intensity of the transmitted light can be controlled such that images can be displayed. Among the various types of LCD devices commonly used, active matrix LCD (AM-LCD) devices having thin film transistors (TFTs) with pixel electrodes connected to the TFTs disposed in matrix form have high resolution and superiority in displaying moving images.
FIG. 1 is a schematic perspective view of an AM-LCD according to the related art. As shown in FIG. 1, an LCD device includes an array substrate 10 and a color filter substrate 20 facing each other and a liquid crystal molecules 30 between the array substrate 10 and the color filter substrate 20. The array substrate 10 includes a first transparent substrate 12, a plurality of gate lines 14 and a plurality of data lines 16 crossing each other and define a plurality of pixel regions “P.” With respect to one pixel region “P,” a thin film transistor (TFT) “Tr” is formed at a crossing of one of the plurality of the gate lines 14 and one of the plurality of data lines 16. A pixel electrode 18 is formed in the pixel region “P” and is connected to the thin film transistor “Tr.” The color filter substrate 20 includes a second transparent substrate 22, a black matrix 25, a color filter layer 26, and a common electrode 28. The black matrix 25 has a lattice shape to shield a non-pixel region (not shown) such as the regions corresponding to the gate and data lines 14 and 16 and the TFT “Tr.” The color filter layer 26 includes red, green and blue sub-color filters 26a, 26b and 26c. Each of the red, green and blue sub-color filters 26a, 26b and 26c is disposed in the pixel regions “P.”
Although not shown, a seal pattern is formed using a sealant to prevent leakage of the liquid crystal molecules 30 at a periphery of a display region (not shown) of the LCD, and a first orientation film is positioned between the common electrode 28 and the layer of liquid crystal molecules 30, and a second orientation film is positioned between the pixel electrode 18 and the liquid crystal layer 30 within the region of the seal pattern to control initial alignment of the liquid crystal molecules. Further, at least one polarizer, which transmits light parallel to polarization axis, may be attached on at least one outer side of the array substrate and the color filter substrate, and a backlight unit is disposed under the one polarizer as a light source.
On/off signals of the TFT “Tr” are sequentially applied to the gate line 14, and then an image signal of the data line 16 is transmitted to the pixel electrode 18 with respect to a selected pixel region “P.” Next, the liquid crystal molecules are driven by a vertical electric field, thereby displaying various image in accordance with change of optical transmittance.
The first and second transparent substrates 12 and 22 are conventionally made of glass. However, as a small-size portable device such as notebook or a personal digital assistant (PDA) has been widely utilized, plastic has been suggested as a plastic substrate because the plastic is lighter, slimmer and more flexible than the glass. The array substrate for an LCD including the TFT is typically manufactured under the high temperature process more than 200 degrees Celsius. However, the plastic substrate cannot be manufactured under a high temperature process of more than 200 degrees Celsius because the plastic substrate is weak against heat and chemical treatment.
To solve these problems, TFTs are formed using an organic semiconductor material under a low-temperature of less than 200 degrees Celsius. In this low-temperature process, the step of forming TFTs may be performed using a coating apparatus that is more inexpensive than a depositing apparatus, thereby reducing process costs. However, glass may be utilized for the array substrate using the organic semiconductor material as well as the plastic substrate. Hereinafter, the method of fabricating the array substrate using the organic semiconductor material at a low-temperature will be explained.
When the array substrate is formed under the low-temperature process of less than 200 degrees Celsius, the electrodes and the lines, including a metallic material, and the passivation layer, including an insulating material, do not substantially affect the electrical characteristics of the TFT, regardless the temperature condition. However, in case of the semiconductor layer including a channel as a moving path of carriers, an amorphous silicon is formed under the low-temperature of less than 200 degrees Celsius, the inner crystal structure of the semiconductor layer is not substantially organized. Therefore, the mobility of the TFT may be hampered. Accordingly, a semiconductor layer of organic semiconductor material is suggested instead of amorphous silicon to solve the mobility problem. However, the organic semiconductor material that is formed by a coating method may be weak against development solution or etchant solution. Therefore, the electrical characteristics of the organic TFT may deteriorate when the organic semiconductor layer is exposed during a developing or coating process.
Accordingly, a top-gate type organic TFT is currently suggested because the organic semiconductor layer, the gate insulating layer and the gate electrode can be simultaneously patterned in the top-gate type organic TFT. In the case of either the top-gate type organic TFT or the bottom-gate type organic TFT, the thickness of the gate insulating layer affects the electrical characteristics of the organic TFT. For example, when the gate insulating layer is formed a thickness of less than 1,000 Å, there is a dielectric breakdown phenomenon that can occur in which the gate insulating layer is ruptured due to the subsequent processing. On the other hand, when the gate insulating layer is formed with an excessive thick thickness, the electrical characteristics of the organic TFT may deteriorate.